Local multivariable solver for optical proximity correction in lithographic processing method, and device manufactured thereby

ABSTRACT

A multivariable solver for proximity correction uses a Jacobian matrix to approximate effects of perturbations of segment locations in successive iterations of a design loop. The problem is formulated as a constrained minimization problem with box, linear equality, and linear inequality constraints. To improve computational efficiency, non-local interactions are ignored, which results in a sparse Jacobian matrix.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/644,790, filed Dec. 22, 2009, now U.S. Pat. No. 8,239,786, whichclaims priority to U.S. Provisional Application No. 61/141,298, filedDec. 30, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to resolution enhancementtechniques for photolithography and more particularly to techniques foroptical proximity correction.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. The apparatus generally comprisesa radiation system for supplying a beam of radiation, a supportstructure for supporting a patterning device, the patterning deviceserving to pattern the beam, a substrate table for holding a substrate,and a projection system for projecting the patterned beam of radiationonto a target portion of the substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, which is alternatively referredto as a mask or a reticle, may be used to generate a circuit patterncorresponding to an individual layer of the IC, and this pattern can beimaged onto a target portion (e.g., comprising part of, one or severaldies) on a substrate (e.g., a silicon wafer) that has a layer ofradiation-sensitive material (resist). In general, a single substratewill contain a network of adjacent target portions that are successivelyexposed. Known lithographic apparatus include so-called steppers, inwhich each target portion is irradiated by exposing an entire patternonto the target portion at once, and so-called scanners, in which eachtarget portion is irradiated by scanning the pattern through theprojection beam in a given direction (the “scanning”-direction) whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system.” The radiationsystem may also include components operating according to any of thesedesign types for directing, shaping or controlling the projection beamof radiation, and such components. The radiation system as well as theprojection system generally comprise components for directing, shapingor controlling the projection beam of radiation. Generally, theprojection system comprises means to set the numerical aperture(commonly referred to as the “NA”) of the projection system. Forexample, an adjustable NA-diaphragm can be present in a pupil of theprojection system. The radiation system typically comprises adjustingmeans for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution upstream of the mask (in a pupil of the radiation system).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure. The lithographicapparatus may also be of a type wherein the substrate is immersed in aliquid having a relatively high refractive index, e.g., water, so as tofill a space between the final element of the projection system and thesubstrate. Immersion liquids may also be applied to other spaces in thelithographic apparatus, for example, between the mask and the firstelement of the projection system. Immersion techniques are well known inthe art for increasing the numerical aperture of projection systems.

A circuit pattern corresponding to an individual layer of an IC devicegenerally comprises a plurality of device patterns and interconnectinglines. Device patterns may comprise features of different spatialarrangement such as, for example, line-space patterns (“bar patterns”),capacitor contact patterns, patterns of contact holes and DRAM isolationpatterns. A feature is not necessarily characterized by a shape whoseline elements define a closed contour. For example, a spatialarrangement of extremities of two neighboring features and a spacebetween the two extremities may also, in the context of the present textand claims, be referred to as a feature.

In the context of the present text and claims, sizes of features arereferred to as those sizes that the features nominally have at substratelevel. At a mask, the size of a feature is M times larger than thenominal size, where M is the magnification of the projection system(typically, |M|=¼ or ⅕). Generally, additional size deviations at themask are introduced to compensate for errors occurring, for example,during projection and exposure of a pattern; such a re-sizing offeatures of the sub-pattern is referred to hereinafter as biasing and/orOptical Proximity Correction (“OPC”). An amount of biasing and/or OPC isalso commonly expressed in terms of a corresponding, nominal amount ofre-sizing at substrate level. The noun “target” when used in expressionssuch as “target features” is indicating that these features havesubstantially a nominal size as desired for the device layer.

One form of OPC is selective bias. Given a CD vs. pitch curve, all ofthe different pitches could be forced to produce the same CD, at leastat best focus and exposure, by changing the CD at the mask level. Thus,if a feature prints too small at the wafer level, the mask level featurewould be biased to be slightly larger than nominal, and vice versa.Because the pattern transfer process from mask level to wafer level isnonlinear, the amount of bias is not simply the product of the measuredCD error at best focus and exposure time and the reduction ratio, butwith modeling and experimentation, an appropriate bias can bedetermined. Selective bias is an incomplete solution to the problem ofproximity effects, particularly if it is only applied at the nominalprocess condition. Even though such bias could, in principle, be appliedto give uniform CD vs. pitch curves at best focus and exposure, once theexposure process varies from the nominal condition, each biased pitchcurve will tend to respond differently, resulting gin different processwindows for the different features. Therefore, the “best” bias to giveidentical CD vs. pitch may even have a negative impact on the overallprocess window, reducing rather than enlarging the focus and exposurerange within which all of the target features print on the wafer withinthe desired process tolerance.

Other more complex OPC techniques have been developed for applicationbeyond the one-dimensional bias example above. A two-dimensionalproximity effect is line end shortening. Line ends have a tendency to“pull back” from their desired end point location as a function ofexposure and focus. In many cases, the degree of end shortening of along line end can be several times larger than the corresponding linenarrowing. This type of line end pull back can result in catastrophicfailure of the devices being manufactured if the line end fails tocompletely cross over the underlying layer it was intended to cover,such as a polysilicon gate layer over a source-drain region. Since thistype of pattern is highly sensitive to focus and exposure, simplybiasing the line end to be longer than the design length is inadequatebecause the line at best focus and exposure, or in an underexposedcondition, would be excessively long, resulting either in short circuitsas the extended line end touches neighboring structures, orunnecessarily large circuit sizes if more space is added betweenindividual features in the circuit. Since one of the key goals ofintegrated circuit design and manufacturing is to maximize the number offunctional elements while minimizing the area required per chip, addingexcess spacing is a highly undesirable solution.

Two-dimensional OPC approaches have been developed to help solve theline end pull back problem. Extra structures (or assist features) knownas “hammerheads” or “serifs” are routinely added to line ends toeffectively anchor them in place and provide reduced pull back over theentire process window. Even at best focus and exposure these extrastructures are not clearly resolved and they alter the appearance of themain feature without being fully resolved on their own. Assist featurescan take on much more aggressive forms than simple hammerheads added toline ends, to the extent the pattern on the mask is no longer simply thedesired wafer pattern upsized by the reduction ratio. Assist featuressuch as serifs can be applied to many more cases than simply reducingline end pull back. Inner or outer serifs can be applied to any edge,especially two dimensional edges, to reduce corner rounding or edgeextrusions. With enough selective biasing and assist features of allsizes and polarities, the features on the mask bear less and less of aresemblance to the final pattern desired at the wafer level. In general,the mask pattern becomes a pre-distorted version of the wafer-levelpattern, where the distortion is intended to counteract or reverse thepattern deformation that will occur during the lithographic process toproduce a pattern on the wafer that is as close to the one intended bythe designer as possible.

In another OPC technique, instead of appending assist structures such asserifs to a feature, completely independent and non-resolvable assistfeatures are added to the mask. These independent assist features arenot intended or desired to print as features on the wafer, but ratherare intended to modify the aerial image of a nearby main feature toenhance the printability and process tolerance of that main feature.Often referred to as “scattering bars,” this type of sub-resolutionassist feature (SRAF) adds yet another layer of complexity to a mask. Asimple example of a use of scattering bars is where a regular array ofnon-resolvable scattering bars is drawn on both sides of an isolatedline feature, which has the effect of making the isolated line appear,from an aerial image standpoint, to be more representative of a singleline within an array of dense lines, resulting in a process window muchcloser in focus and exposure tolerance to that of a dense pattern. Thecommon process window between such a decorated isolated feature and adense pattern will have a larger common tolerance to focus and exposurevariations than that of a feature drawn as isolated at the mask level.

Many of these OPC techniques may be used together on a single mask withphase-shifting structures of different phases added in as well for bothresolution and process window enhancement. The simple task of biasing aone-dimensional line becomes increasingly complicated as two-dimensionalstructures must be moved, resized, enhanced with assist features, andpossibly phase-shifted without causing any conflict with adjoiningfeatures. Due to the extended proximity range of deep sub-wavelengthlithography, changes in the type of OPC applied to a feature can haveunintended consequences for another feature located within half a micronto a micron. Since there are likely to be many features within thisproximity range, the task of optimizing OPC decoration becomesincreasingly complex with the addition of more aggressive approaches.Each new feature that is added to a design has an effect on otherfeatures, which then must be re-corrected in turn, and the results mustbe iterated repeatedly to converge to a mask layout where each featurecan be printed in the manner in which it was originally intended whileat the same time contributing in the proper manner to the aerial imagesof its neighboring features such that they too are printed within theirrespective tolerances.

Due to this complexity and mutual interaction between features, OPCtechnology has become a major field of innovation and many techniqueshave been widely described on how to “segment” or “dissect” the featuresinto a manageable number of edges for co-optimization, how to prioritizethe optimization routines so that the most critical structures are bestprotected from unintended distortion by nearby OPC assist features onneighboring features, how to resolve phase and placement conflictsbetween features, how to trade off computational speed versus ultimateconvergence of the resulting feature to the desired results, and otherdetails of the full implementation of OPC as a manufacturabletechnology.

OPC has generally moved from a rule-based to a model-based approach. Inmodel-based OPC, both the effect of the exposure tool on the aerialimage and the effect of the resist processing are modeledmathematically. In a model based design process, a pre-OPC layout, anOPC technology file, an optical model, and a resist model are obtained.The OPC technology file describes the types of model based OPCtechniques that are to be used, for example linewidth bias corrections,corner rounding corrections, or line end pull back corrections. Theoptical model describes the illumination and projection optics of theexposure tool. The optical model may also include the effect of imaginginto a thin-film resist or the effect of the mask topography. The resistmodel describes the changes in the resist after being illuminated by themask pattern in the exposure tool. An etch model may also be used inthis type of method. The optical, resist, and etch models can be derivedfrom first principles, determined empirically from experimental data, ora combination of both. The models are usually calibrated at the nominalprocess condition. See R. Socha, “Resolution Enhancement Techniques,”Photomask Fabrication Technology, Benjamin G. Eynon, Jr. and Banqiu Wu,Editors, McGraw-Hill, pp. 466-468, 2005. The pre-OPC layout, the OPCtechnology file, and the models are all inputs to the model-based OPCsoftware.

The model-based OPC software segments the features in the pre-OPC layoutinto edge segments and assigns control points to each edge segment. Eachfeature is segmented prior to applying any OPC techniques because eachfeature, even identically-shaped features, will be subject to differentproximity environments. The control points (or evaluation points) arethe locations where CD or edge placement errors (EPE) will be evaluatedduring the OPC design process. The assignment of the control points is acomplex process that depends on the pattern geometry of the pre-OPClayout and the optical model.

The model-based OPC software proceeds by simulating the printed resistimage on the wafer by applying the optical model and the resist model tothe pre-OPC layout. In general, the simulation is performed at thenominal process condition at which the optical model has beencalibrated. The model-based OPC software then generates the contours ofthe simulated resist image by comparing the simulated resist imagevalues to a selected threshold value. The model-based OPC software thencompares the simulated contours with the pre-OPC layout at the controlpoints to determine if the design layout will deliver the desiredpatterning performance. The comparisons are typically quantified as a CDor an EPE at each control point. The model-based OPC software thendetermines whether a figure of merit for the contour metric of each edgesegment is satisfied. In one embodiment, the figure of merit issatisfied when the total error for the contour metric, e.g., CD or EPE,of each edge segment is minimized. In another embodiment, the figure ofmerit is satisfied when the total error for the contour metric of eachedge segment is below a predetermined threshold. If the figure of meritis satisfied the process ends, but if the figure of merit is notsatisfied, the process continues with a new iteration.

If the EPE of the i-th edge segment (E_(i)) is ΔE_(i) determined at thecontrol point CP_(i), the simplest edge correction amount ΔC_(i) is anegation of the error: ΔC_(i)=ΔE_(i). Such a straightforward correctionfunction does not work well for nonlinear processes because changes onthe mask are not linearly reflected in the printed resist image. Toaccount for nonlinearities such as the mask error factor (MEF), aslightly more complicated correction function can be used:ΔC _(i) =−E _(i) /MEF  (1)

In a particular application, the method of calculating the appropriatecorrection tends to be much more complex, and the correction algorithmscan depend on factors such as linewidth error, fabrication process,correction goals, and constraints. See A. K. Wong, ResolutionEnhancement Techniques in Optical Lithography, SPIE Press, pp. 91-115,2001. For example, if it is assumed that there are n edge segments of afeature and one control point for each edge segment, and that the totalcorrection amount for the i-th edge segment is C_(i), the ultimate goalis to solve for C₁, C₂, . . . , C_(n) iteratively, such that thedifference between resist image values RI(CP_(i)) and the predeterminedthreshold values Tat all control points are equal to zero as:RI(CP_(i))−T=0 for i=1, . . . , n, where CP_(i) are the control points.Alternately, one can minimize the function:

$\begin{matrix}{\sum\limits_{i = 1}^{n}\left\lbrack {{{RI}\left( {CP}_{i} \right)} - T} \right\rbrack^{2}} & (2)\end{matrix}$

Next, at each iteration, the model-based OPC software adjusts the entireedge segment E_(i) according to the calculated correction amount ΔC_(i)for all edge segments to produce a post-OPC layout, such that thesimulated resist image contour moves to match the design geometry. Thenthe model-based OPC software simulates a resist image using the post-OPClayout. The resist image contours and error are then calculated for thesimulated resist image produced using the post-OPC layout. Themodel-based OPC software determines whether the total EP error isminimized or below a certain threshold. The total EP error may bedefined as:

$\begin{matrix}{{Error}_{total} = {\sum\limits_{i = 1}^{n}\left( {\Delta\; E_{i}} \right)^{2}}} & (3)\end{matrix}$

Alternatively, the total EP error can be defined as the maximum EP errorof all segments, i.e.:max{|ΔE _(i) |}, i=1, . . . , n  (4)because the OPC goal may be set such that all edge placement errors mustbe below a certain threshold.

A correction amount is determined individually for each edge segment inthe mask layout, without taking into account effects from the movementsof other edge segments in the layout. As feature size decreases, solvingfor the correction amount for individual edge segments experiencesconvergence problems.

A multivariable solver for OPC has been disclosed in U.S. patentapplication Ser. No. 11/764,128, herein incorporated by reference in itsentirety. In the method described, effects on resist image values at aplurality of edge segments of collective movement of edge segments in amask layout are tracked. Control points and edge segments are definedfor a number of contact features. As contact features tend to berelatively small, moving an edge segment affects the resist image valueat that edge segment's control point and also affects the resist imagevalues at the control points of other edge segments of the contactfeature. For contact features that are placed close together in thelayout, moving an edge segment of one contact feature also affects theresist image values at the control points of edge segments inneighboring contact features. Similar effects on resist image values atcontrol points due to movements of neighboring edge segments can beobserved in mask layouts for non-contact layers.

In accordance with an embodiment, the layout is first subdivided intopatches, which are typically 20 μm×20 μm to 60 μm×60 μm in area, beforeapplying OPC. After OPC has been applied, the patches are combinedtogether to produce the final post-OPC layout. Then, a resist image (RI)is simulated using a design layout (pre-OPC layout). The resist imagemay be simulated using a photolithography simulation system such as thatdisclosed in U.S. Pat. No. 7,003,758, the subject matter of which ishereby incorporated by reference in its entirety. Assuming there exist nmovable edge segments, the edge segments in the design layout areperturbed (i.e., moved) by a predetermined distance, which is specifiedby the n×1 vector ΔC⁰, to produce a perturbed layout. By convention,vectorial quantities and matrices are denoted by bold-faced charactersthroughout this document. A resist image is then simulated using theperturbed layout.

A difference between the simulated RI values produced using the designlayout and the simulated RI values produced using the perturbed layoutare determined for each edge segment. These differences are used tocreate an initial matrix, J⁰. The initial matrix is an n×n matrix, wheren is the number of edge segments in the layout, which for some masks canbe more than a million. The initial matrix is a diagonal matrix in whichthe i-th diagonal entry, where i=1, . . . , n, is computed as thedifferences between the simulated RI values of the i-th segment ΔRI_(i)divided by the amount of the perturbation of the i-th segment ΔC_(i) ⁰.Mathematically, the i-th diagonal entry of J⁰ is given by:[J ⁰]_(ii) =ΔRI _(i) /ΔC _(i) ⁰  (n)

In other words, initially it is assumed that the edge segments do notinteract. Changes in each edge segment's RI value are attributed to thatedge segment only. As a diagonal matrix, the off-diagonal elements of J⁰are identically zero. That is, [J⁰]_(ij)=0 for i≠j.

As described in U.S. patent application Ser. No. 11/764,128, acorrection delta vector is determined based on the pseudo inverse of thematrix. A damping factor is applied to the correction delta vector andedge segments are each moved by the damped correction delta. A simulatedresist image is generated using the updated edge segments. Finally, thematrix is updated based on changes in the simulated resist image valuesand the correction delta. At this point, one iteration is complete. Themethod may be further iterated for a selected number of iterations, orthe resulting layout achieves a figure of merit. As an example, thefigure of merit may be satisfied when a sum of edge placement errors isminimized (e.g., by a least squares method) or when such a sum is belowa predetermined threshold. If the figure of merit is not achieved, themethod is re-iterated. Upon completion, the resulting design may be usedto manufacture a mask, or may be used in computer simulations for otherprocess design activities that require a mask pattern as an input.

In this approach, it can occur that a selected number of iterations isinsufficient to estimate all the unknowns in the full Jacobian matrix.Also, depending on the geometry of the mask, the solution may fail toconverge to a complete solution due to the presence of a local minimum.As a result, alternate approaches may be useful.

SUMMARY OF THE INVENTION

One aspect of embodiments of the present invention provides a methodincluding simulating a photolithography process using a mask layout toproduce a first simulated resist image, perturbing each edge segment inthe mask layout by a selected amount to produce an initial perturbedlayout, simulating the photolithography process using the initialperturbed layout to produce a second simulated resist image, determininga difference resist image value between the first simulated resist imageand the second simulated resist image for each edge segment, creating ann×n matrix J such that ΔRI=JΔC, where ΔRI is an n×1 vector of changes inresist image values and ΔC is an n×1 vector of changes in segmentlocations, initializing the matrix J using the difference in resistimage values divided by the perturbed amount, determining a correctiondelta vector ΔC by minimizing |JΔC+RI|²+α|ΔC|² subject to constraintsimposed on ΔC, wherein the correction delta vector includes a correctiondelta value for each edge segment and α is a non-negative scalar,perturbing each edge segment in the perturbed layout by thecorresponding correction delta value in the correction delta vector ΔCto create a further perturbed layout, simulating the photolithographyprocess using the further perturbed layout to produce a third simulatedresist image, using information from the third simulated resist imageand the matrix J to produce an updated matrix J, and updating thecorrection delta vector ΔC by minimizing |JΔC+RI|²+α|ΔC|² subject toconstraints imposed on ΔC, where the updated matrix J is used in theminimization.

Aspects of embodiments of the invention may include a computer-readablemedium encoded with computer-executable instructions for performing theforegoing method or for controlling a computing system for performingthe foregoing method.

Aspects of embodiments of the invention may include a lithography orlitho cell system configured and arranged to be operated in accordancewith the foregoing method. Such a system may incorporate, for example, acomputer programmed to allow a user to control the device in accordancewith the method, or other methods.

Aspects of embodiments of the invention may include a simulation systemconfigured and arranged to be operated in accordance with the foregoingmethod.

Aspects of embodiments of the invention may include a method formanufacturing a mask in accordance with the foregoing method andincluding designing a pattern for a mask, in accordance with theforegoing method and writing a pattern for at least one mask inaccordance with the determined perturbed layout.

Aspects of embodiments of the invention may include a method ofmanufacturing a microelectronic device using a pattern derived accordingto the foregoing method.

Aspects of embodiments of the invention may include a devicemanufactured in accordance with the foregoing method.

Aspects of embodiments of the invention may include imaging a patterndetermined in accordance with the foregoing method.

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 is a schematic illustration of a photolithography system inaccordance with an embodiment of the invention;

FIG. 2 is an illustration showing segments and control points, alongwith a sparse matrix representing the interaction between changes at thecontrol points;

FIG. 3 illustrates a process window constraint in accordance with anembodiment of the present invention;

FIG. 4 illustrates a process window constraint in accordance with anembodiment of the present invention; and

FIG. 5 illustrates a process window constraint in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion,” respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g., having a wavelength in the range of5-20 nm).

The term “patterning device” used herein should be broadly interpretedas referring to devices that can be used to impart a projection beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the projection beam may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the projection beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

Patterning devices may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned.

The support structure supports, i.e., bears the weight of, thepatterning device. It holds the patterning device in a way depending onthe orientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support can beusing mechanical clamping, vacuum, or other clamping techniques, forexample electrostatic clamping under vacuum conditions. The supportstructure may be a frame or a table, for example, which may be fixed ormovable as required and which may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

FIG. 1 schematically depicts a lithographic projection apparatus whoseprocesses may be simulated and/or improved in the methods of theinvention. The apparatus comprises:

-   -   a radiation system Ex, IL, for supplying a projection beam PB of        radiation (e.g., UV radiation or DUV radiation of a wavelength        of less than about 270 nm, such as wavelengths of 248, 193, 157,        and 126 nm radiation). In this particular case, the radiation        system also comprises a radiation source LA;    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g., a reticle);    -   a second object table (substrate table) WT provided with a        substrate holder for holding a substrate W (e.g., a        resist-coated silicon wafer); and    -   a projection system (“lens”) PL (e.g., a quartz and/or CaF₂ lens        system or a catadioptric system comprising lens elements made        from such materials, either suitable for conventional imaging or        suitable for imaging in the presence of an immersion fluid) for        imaging an irradiated portion of the mask MA onto a target        portion C (e.g., comprising one or more dies) of the substrate        W.

As here depicted, the apparatus is of a transmissive type (i.e., has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (with a reflective mask). Alternatively, the apparatusmay employ another kind of patterning device, such as a programmablemirror array of a type as referred to above.

The source LA (e.g., a UV mercury arc lamp, or a DUV excimer laser)produces a beam of radiation. This beam is fed into an illuminationsystem (illuminator) IL, either directly or after having traversedconditioning optics, such as a beam expander Ex, for example. Theilluminator IL may comprise adjustable elements AM for setting the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally comprise various other components, such as an integratorIN and a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g.,with the aid of suitable directing mirrors); this latter scenario isoften the case when the source LA is an excimer laser. The currentinvention and claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of positioner not explicitly depicted in FIG.1 (and interferometers or linear encoder IF), the substrate table WT canbe moved accurately, e.g., so as to position different target portions Cin the path of the beam PB. Similarly, additional positioners (notexplicitly depicted in FIG. 1) can be used to accurately position themask MA with respect to the path of the beam PB, e.g., after mechanicalretrieval of the mask MA from a mask library, or during a scan. Ingeneral, movement of the object tables MT, WT will be realized with theaid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which are not explicitly depicted in FIG. 1.However, in the case of a wafer stepper (as opposed to a step-and-scanapparatus) the mask table MT may just be connected to a short strokeactuator, or may be fixed. Mask MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at once (i.e., a single “flash”) ontoa target portion C. The substrate table WT is then shifted in the xand/or y directions so that a different target portion C can beirradiated by the beam PB; and

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash.” Instead, themask table MT is movable in a given direction (the so-called “scandirection,” e.g., the y direction) with a speed v, so that theprojection beam PB is caused to scan over a mask image; concurrently,the substrate table WT is simultaneously moved in the same or oppositedirection at a speed V=Mv, in which M is the magnification of the lensPL (typically, M=¼ or ⅕). In this manner, a relatively large targetportion C can be exposed, without having to compromise on resolution.

Mask design, for a lithography system as illustrated in FIG. 1,particularly for imaging at very high resolutions, may require OPCelements in order to properly render the desired features. This isparticularly true where device size is smaller than a wavelength of theimaging light. For example, for a modern machine operating at the 32 nmnode, the imaging light may still be 193 nm light from an ArF laser.

As noted above, one approach to designing OPC elements for a mask mayinclude tracking effects on resist image values at a plurality of edgesegments of collective movement of edge segments in a mask layout asdescribed in U.S. patent application Ser. No. 11/764,128. In order toprovide various improvements in functionality, a method in accordancewith an embodiment of the present invention computes an n×n Jacobianmatrix J such thatΔRI=JΔC  (Eqn. 1)where ΔRI is an n×1 vector of changes in RI values between successiveOPC iterations and ΔC is an n×1 vector of changes in segment locationbetween successive iterations.

FIG. 2 illustrates four contact holes separated by a pitch L and eachhaving respectively numbered edges 1 to 16. For purposes ofillustration, the main segment is segment 1 and its nearest neighboringsegments are 2, 3, 4, and 7. Also illustrated in FIG. 2 is the sparsematrix J of dimension 16×16, where n=16 in this case.

The matrix J is a possibly sparse, mostly banded matrix as it can beassumed that segments that are physically distant have minimalinteractions which may be ignored. As illustrated in FIG. 2, a mainsegment affects its neighboring segments according to their physicalproximity. In the example shown, segments 1 (the main segment), 2, 3, 4and 7 (the four nearest neighbors) are assumed to be able to change themain segment's RI value if the segments are moved. Therefore, in thefirst row of the matrix, the elements α₁₁, α₁₂, α₁₃, α₁₄, and α₁₇ do notequal zero, while the remaining elements in the first row are assumed tobe zero.

Assuming that the matrix J is known, the solver determines a correctionamount ΔC for edge segments in the layout at the current OPC iterationby minimizing|JΔC+RI| ² +α|ΔC| ² =ΔC ^(T) J ^(T) JΔC−2RI ^(T) JΔC+RI ^(T) RI+αΔC ^(T)ΔC  (Eqn. 2)subject to the constraints i) ΔC_(min)≦ΔC≦ΔC_(max) and ii) AΔC=b, whereRI is an n×1 vector of RI values, where the superscript T denotes amatrix transpose, ΔC_(min) and ΔC_(max) are n×1 vectors specifying theminimum and the maximum allowable values of ΔC, respectively, A is anr×n matrix and b is an r×1 vector specifying the r equality constraints(e.g., ΔC₁−ΔC₂=0, ΔC₁−ΔC₃=0, etc.), and a is a non-negative scalar. Whenthe matrix J is rank-deficient or near rank-deficient, multiplesolutions might result as the system becomes under-determined; in thiscase, the presence of a prevents the solver from converging toundesirable solutions whose norm |ΔC| is large. In practice, the valuesfor ΔC_(min) and ΔC_(max) may be obtained from the mask design rules orfrom user specifications and may be on the order of, for example, a fewnanometers.

The box constraint on ΔC (i.e., constraint i), above), implements atrust-region or restricted step method to the solution. In this case,the model (the Jacobian J) is known to be accurate as long as the normof the solution |ΔC| stays below a certain value.

The equality condition AΔC=b is imposed if it is desirable to havesymmetry in the mask. Often, mask patterns for contact holes, forexample, will be largely symmetric and this is a valid approach. In analternate approach, computational performance may be greatly improvedthrough the use of substitution in order to reduce a total number ofunknowns. For example, if the original problem is to minimize x₁ ²+2x₂ ²subject to an equality constraint x₁=x₂, then substitution reduces theproblem to one of minimizing 3x₁ ² with no constraints.

Because the system is, in general, nonlinear, the matrix elements J_(ij)are continuously estimated and updated (i.e., at the end of eachiteration) in order to improve convergence of the Jacobian. In thisregard, Broyden's method or a Kalman filter may be applied.

In an embodiment based on Broyden's method, for iteration k−1 we haveRI^(k−1) and J^(k−1). At the following iteration k, the segments move byΔC^(k) and the new values for RI are RI^(k). Broyden's method updatesthe matrix based on differences between the observed change in RI valuesΔRI^(k)=RI^(k)−RI^(k−1) and the predicted change in RI values, JΔC:J ^(k) =J ^(k−1) +[RI ^(k) −RI ^(k−1) −J ^(k−1) ΔC ^(k)](ΔC ^(k))^(T)/ΔC ^(k)|²  (Eqn. 3)where the superscript T denotes a matrix transpose.

In the Kalman filter approach, the non-zero entries of the Jacobianmatrix are estimated using a Kalman filter based on measurement data ΔCand ΔRI. The i-th row in Equation 1 at iteration k:ΔRI _(i) ^(k) ≡RI _(i) ^(k) −RI _(i) ^(k−1)=Σ_(j) J _(ij) ^(k) ΔC _(j)^(k)  (Eqn. 4)resembles the observation equation used in a Kalman filter, when ΔRI_(i)^(k) is considered to be the observed output, the i-th row of J^(k) isconsidered to be the state vector to be estimated, and (ΔC^(k))^(T) isconsidered to be the observation-model matrix which maps the state spaceinto the observed space. In other words, the observation equation usedin the Kalman filter isΔRI _(i) ^(k)=(ΔC ^(k))^(T)(J_(i·) ^(k))^(T) +e ^(k)  (Eqn. 5)where J_(i·) ^(k) denotes the i-th row of the J matrix at iteration kand e^(k) is a scalar noise term with zero mean and variance Σ_(e) ^(k).The system equation used in the Kalman filter isJ _(i·) ^(k) =J _(i·) ^(k−1)+ε^(k)  (Eqn. 6)where ε^(k) is an n×1 vectorial noise term with zero mean and an n×ncovariance matrix Σ_(ε) ^(k). Using the Kalman-filter formulation, then×1 Kalman gain matrix is computed at iteration k asK ^(k)=(P ^(k−1)+Σ_(e) ^(k))ΔC ^(k)[(ΔC ^(k))^(T)(P ^(k−1)+Σ_(ε) ^(k))ΔC^(k)+Σ_(ε) ^(k)]⁻¹  (Eqn. 7)where P^(k−1) is the n×n covariance matrix for the state vector atiteration k−1. The state vector, i.e., the i-th row of J, is updated atiteration k as(J _(i·) ^(k))^(T)=(J _(i·) ^(k−1))^(T) +K ^(k) [ΔRI _(i) ^(k)−(ΔC^(k))^(T)(J _(i·) ^(k−1))^(T)]  (Eqn. 8)Similarly, the covariance matrix for the state vector is updated atiteration k asP ^(k) =[I−K ^(k)(ΔC ^(k))^(T)](P ^(k−1)+Σ_(ε) ^(k))  (Eqn. 9)where I is the n×n identity matrix. In the case that J is sparse, whosei-th row has q≦n non-zero entries, for example, Equations 4 to 9 remainthe same but the dimensions of the matrices are reduced. The dimensionsof the state vector, its covariance matrix, and the Kalman gain matrixare now q×1, q×q, and q×1, respectively.

The sparsity of the Jacobian J can be varied with the iteration number.For example, at the first few iterations where not too much is learnedabout the matrix due to limited measurement data, J can be made quitesparse, even diagonal.

Either approach should be useful for converging on a solution to theminimization problem. In practice, it has been shown that each iterationmay take approximately 0.1 ms of computing time for n=1000. Experimentalresults show that 12 iterations can provide good convergence, thuscomputing time can be about 1 ms. Moreover, even for a much largernumber of iterations (e.g., 30), solutions should take only a few ms.

Additional functionality may be included to allow for additional linearinequality restraints. In particular, process-window constraints may beapplied. In FIG. 3, the requirement that the RI contour at one of theprocess-window conditions must not expand past the points labeledRI_(EPE+,1) and RI_(EPE+,2), which will result in a large positive EPEs,can be stated in as linear inequality constraint: RI_(EPE+)+J_(PW)ΔC≦0,where RI_(EPE+) is the n×1 vector of RI values evaluated at thelocations RI_(EPE+,1), RI_(EPE+,2), etc., J_(PW) is the n×n Jacobianmatrix describing the linear relationship ΔRI_(EPE+)=J_(PW)ΔC at theparticular process-window condition. Similarly, as shown in FIG. 4, therequirement that the RI contour at one of the process-window conditionsmust not shrink beyond the points labeled RI_(EPE−,1) and RI_(EPE−,2),which will result in a large negative EPEs, can be stated in as linearinequality constraint: RI_(EPE−)+J_(PW)ΔC≧0. In addition, functionalitymay be included to allow for mask rule constraints (e.g., minimumspacing, maximum widths, etc.) using the inequality constraint d≦EΔC≦f,where d and f are t×1 vectors and E is a t×n matrix specifying the tlinear inequalities. For example, as shown in FIG. 5, if the minimumspace between the two opposing segments in a mask is minSpace whiletheir original spacing is YY, the inequality becomesYY−(ΔC₁+ΔC₂)>=minSpace, where ΔC₁ and ΔC₂ are the correction deltavalues for the two segments.

In accordance with an embodiment of the invention, the method isperformed by software modules configured to execute the simulationprocesses described herein. The software modules can in turn be embodiedin machine executable instructions stored on a machine readable medium.Modules can be configured to perform the splitting algorithms andadditional modules, either embodied in a common or specialized computingsystem or embodied in a lithographic apparatus itself, can be configuredto operate a lithographic apparatus to perform imaging of devices ontosubstrates using masks designed in accordance with the methods describedherein.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, though reference is madeherein to a computer, this may include a general purpose computer, apurpose-built computer, an ASIC programmed to execute the methods, acomputer array or network, or other appropriate computing device. As afurther example, it is to be understood that the present inventioncontemplates that, to the extent possible, one or more features of anyembodiment can be combined with one or more features of any otherembodiment.

What is claimed is:
 1. A method implemented by a computer for reducingoptical proximity effect in a design layout to be included in a maskused in a lithographic process, the method comprising: dividing patternswithin the design layout into edge segments; identifying a plurality ofneighboring edge segments that interact with each other due to theoptical proximity effect; and computing, by using a computer, an amountof movement of each of the identified neighboring edge segments forreducing the optical proximity effect, the computing includingmaintaining a plurality of model values that can be used to predict achange in an image value associated with each of the identifiedneighboring edge segments given a correction amount of movement whereinthe optimal amount of movement is computed for the identifiedneighboring edge segments in terms of alteration of image values atpredefined locations in the design layout; wherein the plurality ofmodel values are arranged in a matrix form, the matrix being used tocompute the amount of movement; and wherein the matrix comprises asparse matrix, where matrix elements that represent effects ofnon-neighboring edge segments are assumed to be zero.
 2. The method ofclaim 1, wherein the image value is a resist image value.
 3. The methodof claim 1, wherein the computed optimal amount of movement tracks aneffect of collective movement of the identified neighboring edgesegments.
 4. The method of claim 1, wherein computing the movementamount for the edge segments in the design layout includes minimizing anobjective function comprising the sparse matrix.
 5. The method of claim4, wherein the minimization is done in an iterative manner.
 6. Themethod of claim 4, wherein the minimization is subject to one or moreconstraints.
 7. The method of claim 6, wherein a maximum and a minimumvalue of the correction amount are used as constraints.
 8. The method ofclaim 7, wherein one or more of the maximum and the minimum value of thecorrection amount are determined from mask design rules.
 9. The methodof claim 7, wherein one or more of the maximum and the minimum value ofthe correction amount are determined from user specification.
 10. Themethod of claim 7, wherein one or more of the maximum and the minimumvalue of the correction amount are in the order of nanometers.
 11. Themethod of claim 6, wherein a symmetry in the design layout is used as aconstraint.
 12. A method implemented by a computer for reducing opticalproximity effect in a design layout to be included in a mask used in alithographic process, the method comprising: dividing patterns withinthe design layout into edge segments; identifying a plurality ofneighboring edge segments that interact with each other due to theoptical proximity effect; and computing, by using a computer, an amountof movement of each of the identified neighboring edge segments forreducing the optical proximity effect, wherein the computing considers acombined effect of collective movement of the identified neighboringedge segments on a value of a simulated image characteristic at one ormore predefined locations in the design layout, the computing includingmaintaining a plurality of model values that can be used to predict achange in the value of the simulated image characteristic associatedwith each of the identified neighboring edge segments given a correctionamount of movement wherein the optimal amount of movement is computedfor the identified neighboring edge segments in terms of alteration ofimage values at predefined locations in the design layout; wherein theplurality of model values are arranged in a matrix form, the matrixbeing used to compute the amount of movement; and wherein the matrixcomprises a sparse matrix, where matrix elements that represent effectsof non-neighboring edge segments are assumed to be zero.
 13. The methodof claim 12 wherein the simulated image characteristic is a resist imagevalue.
 14. The method of claim 12, wherein computing the movement amountfor the edge segments in the design layout includes minimizing anobjective function comprising the sparse matrix.
 15. The method of claim14, wherein the minimization is subject to one or more constraints.